This invention relates to an underfrequency relay for monitoring the frequency of an alternating current power line and for providing an output signal whenever the frequency decreases below a predetermined value for a specific length of time.
The frequency of an alternating current power system must be maintained within a very narrow range of frequencies. Should the frequency decrease appreciably below 60 Hertz, in the United States, for example, this may be an indication of an abnormal condition requiring a utility company to disconnect or shed some of its load in order to maintain its generators on line, even at the lower frequency.
The measurement of a power line underfrequency has been accomplished in several ways. One technique for measuring the period is to provide an oscillator, the frequency of which is substantially higher than the line frequency being measured, and counting the number of pulses from the oscillator during the period. Should the line frequency decrease, the period will correspondingly increase, and thereafter a high number of oscillator output pulses will be counted. Highly accurate frequency measurements may be made in this manner.